This invention relates to an anisotropic conductive adhesive film to be used in, for example, electrically connecting a liquid crystal display (LCD) to a circuit board.
It has been a practice to connect, for example, a liquid crystal device to an integrated circuit board, etc. by using anisotropic conductive adhesive films.
These anisotropic conductive adhesive films are used in fixing and electrically connecting various terminals to each other, for example, in connecting the connection electrode of a tape carrier package (TCP) or an IC chip to a pattern electrode formed in the glass board of an LCD panel.
In general, an anisotropic conductive adhesive film comprises an insulating binder containing conductive particles As the conductive particles in this case, use is made of those obtained by forming a conductive thin film by for example, nickel- or gold-plating on the surface of particles made of metals (nickel, silver, etc.) or resins.
A fine pitch (pitch: about 60 xcexcm) and a high connective reliability (about 1,000 hours at 85xc2x0 C. under relative humidity of 85%) are required in connecting an LCD panel with the use of a wiring pattern made of aluminum (Al) or chromium (Cr) on which an oxide film is frequently formed on the surface of an electrode pattern.
However, these requirements cannot be satisfied by the prior art.
In the case of an anisotropic conductive adhesive film comprising conductive particles made of a metal (Ni, etc.), namely, electricity passes through an oxide film formed on an electrode pattern but a fine pitch cannot be established due to the irregular particle diameter of the metallic particles. In this case, there arises an additional problem that the small linear expansion coefficient, compared with that of the binder, brings about a low connective reliability after heat aging.
In the case of an anisotropic conductive adhesive film with the use of conductive particles obtained by metal-plating resin particles, on the other hand, a fine pitch can be established on a pattern electrode on which no oxide film is formed. However, such an anisotropic conductive adhesive film suffers from a problem of having a high initial resistance to a pattern electrode on which an oxide film is formed.
The present invention, which has been made to solve these problems encountered in the prior art, aims at providing an anisotropic conductive adhesive film capable of maintaining a high connective reliability on connection electrodes with a fine pitch on which an oxide film is formed.
The present inventors have conducted intensive studies to achieve the above-described object. As a result, they have successfully found out that connection electrodes with a fine pitch each having an oxide film can be surely connected to each other by forming specific projections on the surface of a conductive thin film of conductive particles consisting of resin particles and the conductive thin film formed thereon, thus completing the present invention.
According to a present invention, which has been completed based on the finding as described above, an anisotropic conductive adhesive film for electrically connecting terminals to each other comprises an insulating adhesive formed in a film, and conductive particles dispersed in an insulating adhesive, and consists of resin particles having a conductive thin film on the surface thereof and wherein projections are formed on the surface of the conductive thin film.
In the present invention, projections are formed on the surface of the conductive thin film on resin particles. Under pressure, therefore, the projections of the conductive thin film of the conductive particles break through the oxide films of connection electrodes and thus the connection electrodes can be surely connected electrically to each other.
On the other hand, conductive particles having resin particles as the core are used therein, which makes it possible to achieve a regular particle diameter. When the insulating binder is loosened between the connection electrodes after the aging, moreover, the contact of the projections of the conductive thin film with the connection electrodes can be maintained owing to the elastic rebound of the resin particles, thus ensuring a high conductive reliability.
As described above, the present invention makes it possible to provide an anisotropic conductive adhesive film capable of maintaining a high connective reliability on connection electrodes with a fine pitch on which an oxide film is formed.
In the present invention, it is effective that the resin particles have such a hardness as giving a K value at 10% compressive deformation of from 1xc3x97102 to 2xc3x97103 kgf/mm2.
In the present invention, it is also effective that the resin particles have a recovery ratio from 10% compressive deformation of 5% or above.
In the present invention, it is also effective that the conductive thin film has a compressive elastic modulus of 1.5xc3x97104 kgf/mm2 or above.
When the hardness and the recovery ratio of the resin particles are defined each within a specific range, or the compressive elastic modulus of the conductive thin film is set to a level sufficiently higher than the hardness of the resin particles, the projection can break through the oxide film of the pattern electrode due to the elastic rebound of the resin particles caused by the compressive deformation under pressure. Thus, the connection electrodes can be surely connected to each other via these conductive particles.
In the present invention, it is also effective that 4 to 300, on average, projections are formed on the surface of the conductive thin film.
When 4 to 300 projections are formed on the conductive film, the conductive particles breaking through the oxide film can come into contact with the connective electrodes in a sufficiently large contact area and thus the electrodes can be satisfactorily connected to each other.
In the present invention, it is also effective that the content of the conductive particles is from 1 to 5% by volume.
According to the present invention, the connection electrodes can be more surely connected to each other via the conductive particles.
In the present invention, it is further effective that an insulation layer is formed on the surface of the conductive thin film of the conductive particles.
The constitution according to the present invention makes it possible to prevent electrical short circuit among the conductive particles, even though the conductive particles are used in a rather large amount and condense together.
These and other objects of the invention will become more apparent in the detailed description and examples which follow.